Oil shale is a naturally occurring material that contains a hydrocarbonaceous component referred to as "kerogen" which upon heating releases a hydrocarbonaceous vapor useful as a feedstock in petroleum processing. Both the true oil shales, i.e., those composed principally of silaceous sediments, and "false" oil shales found throughout the western United States (technically these latter materials are a marlstone) are fissile rock which upon crushing and grinding yield a variety of particle sizes ranging from a fine dust to large chunks. The inability of conventional grinding and crushing operations to produce a feed of uniform particle size, has led to downstream processing problems when a fluidized bed is used to pyrolyze the oil shale because such processes are able to tolerate only a limited range of particle sizes. In those processes using a recycled heat transfer material, the solids handling problem is compounded by the increased volume of material.
The use of a partially fluidized bed to control the passage of solids through the pyrolysis zone is one means for extending the ability of the retort to handle a wide range of particle sizes. An example of this type of bed is the staged turbulent bed described in U.S. Pat. No. 4,199,432. However, such improvements still require that the amount and maximum size of non-fluidizable particles be controlled within specified limits. Since the crushing and grinding step in most oil shale processes significantly increases in cost as the usable range of particle sizes narrows, it is desirable to design any process to be able to handle as broad a range of particle sizes as possible.
The present invention is concerned with a modified oil shale retorting process which increases the maximum particle size which can be tolerated in fluidized retorting systems using a particulate heat transfer material.